The CIA will provide "thousands of documents" — most of them classified — to lawmakers, providing them with intelligence about Iraq's weapons of mass destruction (search) program, officials told Fox News on Wednesday.

The data, said to stack at least four-feet high, could be handed over as early as next week, and will provide specific sources and the agency's assessments, both used by Secretary of State Colin Powell to form his presentation to the U.N. Security Council (search) in February.

In that presentation, Powell laid out the administration's intelligence about Iraq's weapons of mass destruction program and ties to terrorism.

The documents will include interviews, including sources' names, telephone and radio intercepts, satellite photos and seized Iraqi documents.

Critics have complained in the past several days about the failure to find weapons of mass destruction so far, and have looked to reports indicating that the Bush administration may have manipulated intelligence to select the portions that best supported going to war with Iraq.

On Wednesday, Undersecretary of Defense for Policy Doug Feith (search) told doubters that the intelligence used was not only credible, but dated back several years.

"These judgments were based on intelligence reports that not only went back years, but pre-dated this administration," Feith said in a rare appearance before reporters. "In February 1998, [former President] Clinton said Iraq continues to conceal weapons of mass destruction and weapons that can deliver them. The intelligence community's judgments on weapons of mass destruction did not undergo major changes between Clinton-Gore and this administration."

But critics have posed the question that if the intelligence was so good, why have weapons inspectors not been successful.

U.S. officials said they are "confident" that weapons of mass destruction will still be found inside Iraq despite the lack of success on that front so far.

According to one senior official, "We are going to find the stuff, but it may take some time."

The first members of the new Iraq Survey Group (search) arrive in Baghdad at the end of the week. The 1,400 investigators and scientists are taking over the search with a focus on finding Iraqis who are leading the way.

The official said problems have hampered the search so far. For instance, many of the suspected weapons of mass destruction sites have been looted, and on at least three occasions documents believed to be tied to Iraq's program were destroyed before teams arrived.  Scraps were found at those sites. The officials also said a climate of fear in Iraq remains that has made it difficult for U.S. teams to encourage scientists to come out and talk.

Critics have said the excuses are piling up along with the bad intelligence.

Sen. Pat Roberts, R-Kan., chairman of the Senate Intelligence Committee, said he would not commit to holding hearings on the topic of U.S. intelligence, but would review the materials.

"As part of its continuing oversight of the U.S. intelligence community's activities and programs, the Senate Select Committee on Intelligence will conduct a thorough review of the documented intelligence underlying the assessments, which determined the existence of and the threat posed by Iraq's weapons of mass destruction programs," he said in a statement.

The top Democrat on the committee, Sen. Jay Rockefeller of West Virginia, said the panel should begin a formal investigation of prewar intelligence, not just review the CIA's documents.

A senior official said the documents being sent to Capitol Hill will "prove to congressmen who have been making these statements as of late that we weren't making this stuff up."

Fox News' Bret Baier contributed to this report.